1. Field of The Invention
The present invention relates generally to a high frequency welding system. Particularly the present invention relates to a high frequency seam welding system which can control a manufacturing process for providing sealing by welding between opposite sides of a material being formed into a cylindrical shape such as piping, for example.
2. Description of The Prior Art
Production systems for piping and other tubular members are known in which a workpiece is fed from a roll of metal strip in a tubular formation such that opposite sides thereof are positioned adjacently. An upset pressure is supplied to butt the sides of the workpiece together at a jointing point and supplying a high frequency electrical power to the workpiece to weld the opposite side surfaces at a welding point. It is conventional practice to adjust the intensity of the welding heat generated at, and near the jointing point by controlling the high frequency power to the workpiece based upon various conditions which are monitored by sensors during the production process. However, it is very difficult to monitor each of the many conditions which may affect welding heat during such a production process.
FIG. 7 shows an electromagnetic inductance type conductive portion for a welding system, FIG. 8 shows contact type conductive portion of conventional seam welding systems for forming cylindrical members. According to the drawings, a material 1 for forming a pipe undergoes a multistage process for rolling the material 1. When the material 1 is first rolled to approach a cylindrical shape, a V-shaped gap, or seam, 2 is formed along one side of the rolled material 1 as the material 1 is rolled in the direction of the arrow A of FIGS. 7 and 8. The V-shaped gap is known as a V throat. According to the electromagnetic inductance method of FIG. 7, a heating coil 3a is powered from a high frequency power source through a power circuit. The welding heat under which the workpiece, or material 1 is welded, at a welding point 1a, is determined by the level of power applied to the heating coil 3a. According to the contact type system of FIG. 8, a high frequency current I is applied from an electrical source, or work coil 3 which is connected to opposed sides 2a and 2b of the V-shaped gap 2 via electrodes 4a and 4b respectively.
After either of the above described steps, the pipe material 1 is put between squeeze rollers 5a and 5b which apply an upset pressure in the directions of arrows B and C of FIGS. 7 or 8 for joining the opposed sides 2a and 2b for continuously forming a welded line seam 10.
FIG. 14 shows a cross section of end pieces 2a and 2b of a seam to be joined by welding. Heated portions of the seam are shown in the drawing by hatching. Referring to FIG. 14(A), the flat ends of each side 2a, 2b of the seam 10 to be joined are heated. According to this arrangement wherein a welding current I is applied to sides of the seam 10, a proximity effect is conspicuous between the opposed ends 10a, 10b of the seam 10. FIG. 14(B) is a close-up view of a thickness portion of the end pieces 10a and 10b of the seam 10, as can be seen from the drawing, according to this effect, a current I is stronger at a corner portion of the ends 10a and 10b, thus heating is stronger at each corner of each of the ends to be joined. Thus, as seen in FIG. 14(C), when pressure is applied by the squeeze rollers 5a and 5b for joining the ends 10a and 10b of the seam 10, a center portion thereof is heated less than the corner portions which can lead to spattering of heated metal when the ends 10a, 10b are joined under pressure and may further lead to formation of `pinholes` along the seam thus degrading the quality of welded seam.
In order to deal with the problem outlined above, Japanese Patent Application 2-139244 discloses an alternative type of conventional seam welding system as shown in FIG. 9. According to this arrangement, before the seam 10 proceeds to the seam welding portion 6 of the apparatus, it is preheated at a preheating portion 7. The preheating portion includes a guide means 8 and a second electrical source 9 for supplying mid and low frequency current to the seam 10. The guide means is interposed between an inner and outer surface of the material 1 for supplying relatively low frequency heating to a core, or center portion of the ends 10a and 10b of the seam 10 allowing substantially even heating of the core and corner portions of ends 10a, 10b to be achieved at the welding stage for forming the seam 10.
According to the above arrangement, a relatively high cost is incurred due to the more complex apparatus and, according to the application of high and lower frequency currents for heating, a high output electrical source is required. Such high output sources are subject to current variation at high frequencies.
FIG. 19 shows a induction heating circuit for such conventional welding systems. The circuit includes a hot cathode electron tube 40, and an oscillator circuit 50 therefor, a direct current voltage Edc is required for causing oscillation of the electron tube 40. A three phase voltage e.sub.1 is introduced through a stepdown transformer TR1 to be limited to a withstand threshold of a thyristor 100, the thyristor 100 regulates the output which is supplied to an amplifying transformer TR2 and is then supplied to a three phase rectifier circuit 20 and a filter 30 is provided for smoothing.
Further shown in FIG. 19 is a filament circuit 70 for the electron tube 40. A single phase source voltage e.sub.2 is supplied to the filament circuit 70 through an AVR (Automatic Voltage Regulator). The stabilized output from the AVR is supplied to a filament transformer TR3 and the output of the transformer TR3 is supplied to the filament 40a of the electron tube 40 for heating thereof. Also associated with the electron tube 40 is a grid bias circuit 80, capacitors Ct.sub.1, Ct.sub.2 and feedback capacitors Cg.sub.1 and Cg.sub.2.
The above described type of circuit is subject to ripple current which requires provision of a filter. However, for effectively smoothing such ripple current, a large capacity choke coil and a condenser must be added, increasing the size, weight and complexity of such a circuit.
Further, for low frequency ripple a filter for higher harmonic frequencies is needed, and the size and cost of the circuit is increased. In addition, the thyristor 100 provided for voltage regulation has too slow a response to effectively deal with such ripple current.
When such as circuit as the above-described is used as a heating circuit for induction welding, for example, ripple current present in the circuit creates fluctuation in the high frequency output voltage in the emissions of the electron tube 40 causing unevenness in the resulting welds.
For monitoring such a welding system, one of the following three methods are conventionally employed; 1) visual monitoring by a system operator, 2) measuring irradiated temperature of the welding operation, 3) electronically detecting oscillation frequency variation for discriminating excess applied heating 4) monitoring the shape and projection of a welding bead;